insufficient fuel... run it with the float chamber lid off and see if the float still stays floated.... unless you have nice paintwork as fuel might slop everywhere. Edit: ensure you don't set light to your shed whilst doing that.

The mag could be playing up after a short while or it could be the plug playing up as it warms up.

Is it running too lean on the mixture? When you start it does it black smoke, if so then just screw the mixture wheel in until it just stops smoking and that should be correct. One of my D's would start easily without any smoke but would cut out after several minuites, I had to rotate the mixture control another turn anticlockwise and it now runs fine.